For producing bent glass sheets having a good principal curvature conformity and a satisfactory optical quality, it is important that the glass sheet is correctly positioned before being taken up by the bending and/or heat treatment tools. This is why there are usually glass sheet orientation readjustments within the furnace in the plane of the conveyor rollers and normally frontally, i.e. acting on the "leading" edge of the sheet with respect to its travel axis and/or laterally, i.e. by acting on one or other of the edges substantially parallel to said axis. The nearer said readjustment takes place at the end of the furnace, the more it is effective, because the risk of any further disorientation of the sheets before being taken up on leaving the furnace is more greatly reduced. However, such a `tardy` readjustment significantly increases the risk of the sheets being marked when very close to their final softening point, because it involves both a direct contact between the repositioning members and one or more edges of the sheets and a friction between the lower surface of the sheets and the conveyor rollers.
There are two types of repositioning members, namely "static" and "dynamic" members.
Firstly, EP-A-389 317 discloses an apparatus which can be referred to as "static", which has frontal abutments, e.g. two such abutments, interposed on the path of the glass sheets in the furnace and which when successively struck by the leading edge of each of them, retract as soon as the double contact between the sheet and the two abutments is detected. Thus, the repositioning time is reduced to that strictly necessary, in order to limit the glass/roller friction time. However, by stopping the sheet, this type of static apparatus induces frictions which, even if acceptable up to the center of the furnace, i.e. at a glass temperature below 500.degree. to 550.degree. C., would produce marks on the lower surface of the glass sheets at positions in the furnace beyond this and at higher glass temperatures.
Thus, another type of repositioning member has been proposed and which is referred to as "dynamic", to the extent that such members perform a movement and accompany each sheet over a certain distance whilst reorienting it. Reference can be made in this connection to the frontal recentering apparatus of EP-A-389 316 and the lateral repositioning apparatus of EP-A-367 670. This makes it possible to allow high production rates, whilst simultaneously significantly reducing the intensity of the friction between the glass at the rollers, there being a reduced speed difference during repositioning between the sheets and the rollers, which have a constant rotation speed. Therefore such a dynamic apparatus can be used right up to the end of the furnace.
However, although it has good performance characteristics, it is relatively expensive and has a complex construction, because it requires a detection of the sheets upstream thereof, a good synchronization between the movement of the successive sheets and that of the repositioning members, as well as a systematic regulation between each production series of bent glass sheets. Its use also makes it necessary to reserve within the furnace an area allowing the movement of such a moving member, while having openings therein which are inappropriate for regulating the temperature of the furnace enclosure.
The aim of the present invention is therefore to permit the use of the much simpler static repositioning apparatuses all along the furnace, but without causing any marking on the glass sheets so as to cause any deterioration in their optical quality.
The inventors have found that any frontal repositioning stops or at least decelerates the glass sheet, whereas the rollers continue to rotate in a uniform manner and any lateral repositioning leads to the skidding thereof on the rollers with respect to its travel axis. As stated hereinbefore, this friction increases when using static apparatuses and on approaching the furnace outlet.
Up to now, a conventional means for minimizing such friction was to cover the rollers located in the repositioning area with braided silica wire sheaths. However, their use is not completely satisfactory for several reasons.
Firstly, these silica wires oxidize relatively rapidly at high temperature, even when located at the start or in the center of the furnace. In addition, such sheaths have a mediocre mechanical strength, which frequently leads to local breakages of a few wires and these are propagated very rapidly over the entire sheath length, so that locally the glass sheet rests on a bare roller. As a result, it is necessary to replace the sheaths very frequently. Further, the braided texture of the sheaths, which is similar to a fabric, offers a smooth surface, but has a rather limited "softness" in its thickness. Therefore contact between the glass and silica is not of an optimum nature.
Thus, there remains a need for coverings for conveyor rollers in a glass heat-treatment furnace which are free of the above-described drawbacks. There also remains a need for a glass heat-treatment furnace which contains rollers covered with such coverings and a method for heating glass utilizing such a furnace.